The present invention relates to a composite diffusion type nitriding method, a composite diffusion type nitriding apparatus using the same and a method for producing a nitride, which are especially suitable for nitriding tools, such as general tools and molds, which require abrasion resistance, and machine parts and molds made of a material difficult for nitriding, such as austenitic stainless steel.
A nitriding method has generally been known as a surface hardening method of a metal member. The nitriding method has advantages such that since the nitriding method requires a processing temperature lower than that in a hardening method by cementation, less deformation and strain occur in the metal member, and further since an obtained hardening layer is extremely hard, the hardening layer has excellent abrasion resistance and corrosion resistance.
Heretofore, as nitriding methods of this type, a gas nitriding method, salt-bath nitriding method and ion nitriding method have been known. However, in the salt-bath nitriding method, since cyanic salt is used, a working environment is bad and a treatment of a waste liquid requires a huge cost. Thus, the salt-bath nitriding method is not practical. The ion nitriding method using an electric discharging phenomenon in a vacuum condition is hopeful in the future, but there is a limitation in a shape and the like of a material to be nitrided at this stage.
Contrary to these methods, the gas nitriding method has been established as a practical method, and also in the future, it is supposed that the gas nitriding method will take the first place in the nitriding methods. In the gas nitriding method, an ammonia gas is contacted with a surface of heated steel, so that the ammonia gas is decomposed by a catalytic action to form active atomic nitrogen, and active atomic nitrogen is absorbed into the surface of steel to thereby produce nitride with iron contained in steel.
However, the gas nitriding method as described above has the following disadvantages.
First, with respect to a material difficult for nitriding, such as austenitic stainless steel, the nitriding method itself is difficult.
Also, with respect to a material to be nitrided having a special shape, an embrittlement layer (which is also called as white layer or .epsilon. layer) and incomplete nitriding are liable to occur. More specifically, with respect to a material to be nitrided having a special shape, such as a tool or mold having a sharp edge, a nitriding effect for the edge portion is accelerated more than other portions having a large mass, so that the edge portion is liable to have an embrittlement layer. The embrittlement layer has a nature of becoming thick in proportion to the thickness of the hardened layer. Therefore, when the hardened layer is made thick, the edge is liable to break off, and abrasion resistance is also decreased.
In order to prevent these defects, in case the embrittlement layer is designed as a portion to be polished beforehand, a polishing work after a nitriding treatment requires a great labor and time, and waste of a material and nitriding gas is increased. On the other hand, with respect to a material to be nitrided having a special shape, such as a machine part with a small hole in a long shaft, since the nitriding gas does not fully enter inside the small hole, the nitriding in the small hole portion may become incomplete. Especially, in case the small hole has one end which is closed, the nitriding is still more difficult.
Furthermore, with respect to not only the material to be nitrided having the above described special shape but also a material to be nitrided having a normal shape, there are problems to be solved as described hereinbelow. First, the gas nitriding itself basically takes a long time, so that a processing efficiency is poor, and it is very difficult to improve an operation rate of a furnace and a cost performance of a product. Thus, a using amount of a nitriding gas is increased. Further, since a slight error in setting various conditions with respect to the nitriding results in a large error accumulated for a long time, and there is another problem in adjusting suppression of an embrittlement layer.
The present invention has been made in view of these problems mentioned above.
An object of the present invention is to provide a composite diffusion type nitriding method for effectively nitriding a material, even for a material difficult for nitriding or having a special shape; and with respect to a material to be nitrided having a normal shape, a stable nitriding layer can be formed by a simple method with high efficiency without imposing severe conditions.
Another object of the present invention is to provide a composite diffusion type nitriding apparatus for effectively nitriding a material with a simple structure.
A further object of the present invention is to provide a method for producing a nitride, which can be simply and effectively prosecuted.
Further objects and advantages of the invention will be apparent from the following description of the invention.